1. Field of the Invention
The inventions disclosed and taught herein relate generally to processes for the collection of liquid sulfur and removal of hydrogen sulfide (H2S) and related gases by degassing liquid sulfur. More specifically, the inventions disclosed herein are related to processes for the collection of liquid sulfur produced in a Claus unit or similar sulfur recovery unit while simultaneously degassing the sulfur and removing the liquid sulfur produced, preferably using an above-ground system that includes the use of a liquid jet pump or eductor and the pumping of the sulfur to a storage vessel, using a portion of the liquid sulfur as a jet or eductor motive fluid.
2. Description of the Related Art
The Claus process is a well known process for producing elemental sulfur by reacting hydrogen sulfide and sulfur dioxide. Typically, hydrogen sulfide from gas plants, sour gas field developments, in refinery product gases, or from any other similar facilities which produce H2S is partially combusted to produce sulfur dioxide, which then reacts with the unburned hydrogen sulfide to yield sulfur and water. The sulfide is condensed and recovered. Suitable catalysts may also be used to promote the sulfur production.
The basic chemistry of the Claus process is as follows:H2S+ 3/2O2→SO2+H2O  (1)2H2S+SO2→⅜S8+2H2O  (2)Hydrogen sulfide is soluble in liquid sulfur. The dissolved hydrogen sulfide may react with the sulfur diradical chain species to form hydrogen polysulfides, as follows:H2S+Sx-1→H2Sx  (3)The conversion rate of hydrogen polysulfide back to hydrogen sulfide is generally very low.
One problem thus associated with the Claus process is the presence in the produced liquid sulfur of dissolved hydrogen sulfide. The liquid sulfur removed from the process gas stream in each condenser will contain dissolved H2S in the form of hydrogen polysulfide (H2Sx) and H2S in equilibrium with the process gas. If this dissolved H2S is not removed, it can evolve from the sulfur product during storage and/or transportation, creating potentially hazardous conditions. Degassing the sulfur to reduce the H2S content considerably reduces the potential hazards, and also improves the performance of sulfur forming and grinding processes. Numerous degassing processes involve the agitation of the liquid sulfur and removal of the evolved H2S with a sweep gas. Generally, air is used as the sweep gas since oxygen helps to release the H2S from the polysulfide molecule. The main parameters which are typically taken into consideration for the sulfur degassing are the total hydrogen sulfide and hydrogen polysulfide content in the sulfur to be degassed, as well as the properties of these components which are dissolved in the sulfur. As indicated above, the hydrogen sulfide contaminates the products and may pose serious hazards. For example, molten sulfur from conventional sulfur recovery sources may typically contain as much as 700 parts per million by weight (ppmw as H2S) of dissolved, free H2S and hydrogen polysulfides. At these levels, hydrogen sulfide may create nuisance odors in the vicinity of the liquid sulfur, and may also reach toxic levels when loading and unloading the sulfur. Furthermore, when storing liquid sulfur in tanks or other vessels, a threat exists that the lower explosive limit of hydrogen sulfide in air may be reached.
Typically, sulfur produced from a Claus sulfur plant may contain from about 150 to about 450 ppmw (parts per million by weight) of equivalent hydrogen sulfide which may evolve with time in the storage facilities such as pits, tanks, tank trucks, and/or tank cars and accumulate in the vapor phase creating unsafe conditions. For instance, hydrogen sulfide can escape out of the storage device resulting in a release to the atmosphere. Hydrogen sulfide in the vapor phase of a storage device may build up to a level exceeding its lower explosive limit and cause an explosion. Sulfur storage explosions have occurred in the industry.
To eliminate these and other problems, a number of methods have been suggested or developed that remove hydrogen sulfide from liquid sulfur. Release of dissolved hydrogen sulfide has been carried out by agitating the liquid sulfur by various means, including mechanical agitation, spraying, bubbling air through the liquid sulfur, and by circulating the sulfur over a stripping column. The released hydrogen sulfide is often removed from the pit gas space by a “sweep gas,” which typically may be any suitable gas such as air, Claus tail gas, carbon dioxide, steam, or an inert gas such as argon or nitrogen.
For example, U.S. Pat. No. 5,080,695, issued in 1992 to Kassarjian, describes a method of removing hydrogen sulfide from liquid sulfur wherein the liquid sulfur is caused to flow continuously through a seal pot where it is contacted by a counter-flowing inert gas, thereby stripping hydrogen sulfide from the sulfur. The inert gas, which is typically nitrogen, flows back to the Claus process. One problem with the method described in this patent is that selection of the degassing agent is restricted to inert gases which have been demonstrated to be less effective degassing agents than oxygen-containing gas streams, such as air. Since the spent gas is returned to the overall, primary process in the '695 patent, an oxygen containing gas, such as air, cannot be used due to adverse process impact and safety concerns.
It is a disadvantage of some modern, commercial degassing processes that they require large, complex and, accordingly, expensive equipment. For example, in one process, known as the ‘Shell process,’ degassing takes place in a storage tank or sulfur pit equipped with vertical stripping columns, where liquid sulfur is vigorously agitated by bubbling sparged air through the column at atmospheric pressure in order to induce sulfur recirculation. The stripping columns are open at the top and bottom to allow the sulfur to circulate at a rate of a few hundred times per hour. The sparge air, together with an additional flow of air, is then used as a low pressure sweep gas to displace the gases produced by the degassing process. The low pressure gases so produced are then fed to an incinerator where the H2S is oxidized to SO2 and released to the atmosphere. Depending on the design, a liquid or gaseous catalyst, such as ammonia, ammonium thiosulfate, urea, morpholine, or an alkanol amine may be added to accelerate the decomposition of the polysulfide into H2S.
U.S. Pat. No. 5,632,967 to Goar, Allison & Associates describes what is known in the industry as the “D'GAASS” process. This patent discloses a process for degassing liquid sulfur under pressure. According to the specification, a stream of liquid sulfur containing polysulfides and hydrogen sulfide and a stream of an oxidizing gas are introduced into a vessel and intimately mixed to provide intimate contact between the two streams. The streams are passed through the vessel at a pressure of at least about 40 psig and at a temperature and for a residence time sufficient to remove substantially all of the polysulfides and hydrogen sulfide from the liquid sulfur.
An alternative process, described by the Societe Nationale Elf Aquatane and known as the ‘SNEA process’, catalytically accelerates polysulfide decomposition using ammonia or a variety of heterocyclic amines and an optional surfactant, as described in U.S. Pat. Nos. 4,849,204; 5,004,591 and 5,030,438. In this process, degassing takes place by repeated circulation and spraying of the liquid sulfur into the sulfur pit. Release of dissolved H2S is achieved by spraying liquid sulfur through jets at a specific velocity. After the H2S gas is released, it is removed by a sweep gas and fed to an incinerator. Both the stripping columns used in the Shell process and the circulation/spraying equipment used in the SNEA process are costly and require a large amount of space. Further, it is a disadvantage of both processes that they require the additional step of having to incinerate the H2S-containing sweep gases. In recent years, the catalysts used in these processes have been banned for use by many countries around the world, due to human safety and handling concerns and more stringent environmental regulations.
Other processes which have been described include the use of a fixed-bed alumina catalyst optionally impregnated with an inorganic metal sulfide, or a cobalt-molybdenum impregnated alumina catalyst to catalyze the oxidation of polysulfide in combination with air-stripping.
It is another disadvantage of modern, commercial degassing processes that they require a relatively long retention time in the sulfur pit. For example, the Shell process described above typically requires the liquid sulfur to be recirculated through the stripping columns for about twenty to about twenty-four hours, while the SNEA process typically requires the liquid sulfur to be recirculated through the spray jets for about twenty-four to about thirty hours.
Presently, most industrial facilities remove hydrogen sulfide from the liquid sulfur by injecting a gas, such as air, nitrogen or steam in the pit where liquid sulfur is stored. Some have installed elaborately designed nozzles inside the pit itself for enhanced gas dispersion within the pit.
However, installation or replacement of air sparging or mixing equipment in a storage device such as a pit requires emptying out the entire sulfur inventory. This typically requires shutting down the entire sulfur unit for several days because of the highly corrosive, and pyrophoric environment in the pit. The pit should be depleted of any sulfur and made safe before human personnel may enter and make the installation. Other problems exist with known methods for reducing the amount of hydrogen sulfide in liquid sulfur. Further, several of the degassing processes described above have common problems and similar limitations inherent with conventional sulfur collection systems, such that as sulfur flows by conventional gravity methods from condensers through the sulfur seal or similar device to a storage vessel, they may suffer from additional disadvantages such as conduit plugging, gravity flow constraints, and hydraulic limitations for sulfur collection, necessitating location of the collection pit or vessel near the sulfur plants and below grade, with limited flexibility for sulfur collection.
The inventions disclosed and taught herein are directed to an improved system for liquid sulfur collection from process gas streams with a degassing process, and methods for such improved sulfur collection using these systems.